Zero-crossing amplitude shift keying demodulation

ABSTRACT

Systems, methods and apparatus for wireless charging are disclosed. A charging device has a resonant circuit comprising one or more transmitting coils, a driver circuit configured to provide a charging current to the resonant circuit, a zero-crossing detector configured to provide a zero-crossing signal that includes edges corresponding to transitions of a voltage measured across the resonant circuit through a zero volt level or corresponding to transitions of a current in the resonant circuit through a zero ampere level and an Amplitude Shift Keying demodulator. The demodulator may be configured to receive a plurality of samples of voltage or current in the resonant circuit captured at times determined by the edges included in the zero-crossing signal, and demodulate a modulated signal obtained from the charging current using the plurality of samples of voltage or current in the resonant circuit.

TECHNICAL FIELD

The present invention relates generally to wireless charging ofbatteries, including batteries in mobile computing devices, and moreparticularly to demodulating signals received from a device beingcharged during a charging operation.

BACKGROUND

Wireless charging systems have been deployed to enable certain types ofdevices to charge internal batteries without the use of a physicalcharging connection. Devices that can take advantage of wirelesscharging include mobile processing and/or communication devices.Standards, such as the Qi standard defined by the Wireless PowerConsortium enable devices manufactured by a first supplier to bewirelessly charged using a charger manufactured by a second supplier.Standards for wireless charging are optimized for relatively simpleconfigurations of devices and tend to provide basic chargingcapabilities.

Improvements in wireless charging capabilities are required to supportcontinually increasing complexity of mobile devices and changing formfactors. For example, there is a need for improved communication betweendevices engaged in wireless power transmission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a charging cell that may be employed toprovide a charging surface in accordance with certain aspects disclosedherein.

FIG. 2 illustrates the arrangement of power transfer areas provided by acharging surface that employs multiple layers of charging cellsconfigured in accordance with certain aspects disclosed herein.

FIG. 3 illustrates a wireless transmitter that may be provided in acharger base station in accordance with certain aspects disclosedherein.

FIG. 4 illustrates a phase-modulated wireless charger configured inaccordance with certain aspects of this disclosure.

FIG. 5 illustrates an example of a pulse-width modulation chargerconfigured in accordance with certain aspects of this disclosure.

FIG. 6 illustrate the operation of the pulse-width modulation charger ofFIG. 5.

FIG. 7 illustrates an example of a wireless charging system that employsa class-D wireless transmitter configured in accordance with certainaspects disclosed herein.

FIG. 8 illustrate the operation of the class-D wireless transmitter ofFIG. 7.

FIG. 9 illustrates zero-crossing, slotted foreign object detection inaccordance with certain aspects of the disclosure.

FIG. 10 illustrates a wireless charging system that employszero-crossing detection to obtain measurements at one or more points ineach cycle of current or voltage in a resonant circuit in accordancewith certain aspects of the disclosure.

FIGS. 11 and 12 illustrate phase-based ASK demodulation that supportsusing zero-crossing detection in a wireless charging system configuredin accordance with certain aspects of the disclosure.

FIG. 13 illustrates one example of an apparatus employing a processingcircuit that may be adapted according to certain aspects disclosedherein.

FIG. 14 illustrates a method for operating a charging device inaccordance with certain aspects of this disclosure.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various configurations and isnot intended to represent the only configurations in which the conceptsdescribed herein may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof various concepts. However, it will be apparent to those skilled inthe art that these concepts may be practiced without these specificdetails. In some instances, well known structures and components areshown in block diagram form in order to avoid obscuring such concepts.

Several aspects of wireless charging systems will now be presented withreference to various apparatus and methods. These apparatus and methodswill be described in the following detailed description and illustratedin the accompanying drawing by various blocks, modules, components,circuits, steps, processes, algorithms, etc. (collectively referred toas “elements”). These elements may be implemented using electronichardware, computer software, or any combination thereof. Whether suchelements are implemented as hardware or software depends upon theparticular application and design constraints imposed on the overallsystem.

By way of example, an element, or any portion of an element, or anycombination of elements may be implemented with a “processing system”that includes one or more processors. Examples of processors includemicroprocessors, microcontrollers, digital signal processors (DSPs),field programmable gate arrays (FPGAs), programmable logic devices(PLDs), state machines, gated logic, discrete hardware circuits, andother suitable hardware configured to perform the various functionalitydescribed throughout this disclosure. One or more processors in theprocessing system may execute software. Software shall be construedbroadly to mean instructions, instruction sets, code, code segments,program code, programs, subprograms, software modules, applications,software applications, software packages, routines, subroutines,objects, executables, threads of execution, procedures, functions, etc.,whether referred to as software, firmware, middleware, microcode,hardware description language, or otherwise. The software may reside ona processor-readable storage medium. A processor-readable storagemedium, which may also be referred to herein as a computer-readablemedium may include, by way of example, a magnetic storage device (e.g.,hard disk, floppy disk, magnetic strip), an optical disk (e.g., compactdisk (CD), digital versatile disk (DVD)), a smart card, a flash memorydevice (e.g., card, stick, key drive), Near Field Communications (NFC)token, random access memory (RAM), read only memory (ROM), programmableROM (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM),a register, a removable disk, a carrier wave, a transmission line, andany other suitable medium for storing or transmitting software. Thecomputer-readable medium may be resident in the processing system,external to the processing system, or distributed across multipleentities including the processing system. Computer-readable medium maybe embodied in a computer-program product. By way of example, acomputer-program product may include a computer-readable medium inpackaging materials. Those skilled in the art will recognize how best toimplement the described functionality presented throughout thisdisclosure depending on the particular application and the overalldesign constraints imposed on the overall system.

Overview

Certain aspects of the present disclosure relate to systems, apparatusand methods applicable to wireless charging devices and techniques.Charging cells may be configured with one or more inductive coils toprovide a charging device that can charge one or more deviceswirelessly. The location of a device to be charged may be detectedthrough sensing techniques that associate location of a device tochanges in a physical characteristic centered at a known location on asurface of the charging device. Sensing of location may be implementedusing capacitive, resistive, inductive, touch, pressure, load, strain,and/or another appropriate type of sensing.

In one aspect of the disclosure, an apparatus has a battery chargingpower source, a plurality of charging cells configured in a matrix, afirst plurality of switches in which each switch is configured to couplea row of coils in the matrix to a first terminal of the battery chargingpower source, and a second plurality of switches in which each switch isconfigured to couple a column of coils in the matrix to a secondterminal of the battery charging power source. Each charging cell in theplurality of charging cells may include one or more coils surrounding apower transfer area. The plurality of charging cells may be arrangedadjacent to a surface of the device without overlap of power transferareas of the charging cells in the plurality of charging cells. Devicesplaced on the surface may receive power that is wirelessly transmittedthrough one or more of the charging cells.

In some instances, the apparatus may also be referred to as a chargingsurface. Power can be wirelessly transferred to a receiving devicelocated anywhere on a surface of the apparatus. The devices can have anarbitrarily defined size and/or shape and may be placed without regardto any discrete placement locations enabled for charging. Multipledevices can be simultaneously charged on a single surface. The apparatuscan track motion of one or more devices across the surface.

Certain aspects disclosed herein relate to improved wireless chargingtechniques. In various aspects of the disclosure, a method for operatinga charging device includes providing a charging current to a resonantcircuit when a receiving device is present on a surface of the chargingdevice, providing a zero-crossing signal that includes edgescorresponding to transitions of a voltage measured across the resonantcircuit through a zero volt level or to transitions of a current in theresonant circuit through a zero ampere level, capturing a plurality ofsamples of voltage or current in the resonant circuit at timesdetermined by the edges included in the zero-crossing signal, anddemodulating a modulated signal obtained from the charging current usingthe plurality of samples of voltage or current in the resonant circuit,wherein the modulated signal is modulated using Amplitude Shift Keying.

Charging Cells

According to certain aspects disclosed herein, a charging device may beprovided using charging cells that are deployed adjacent to a surface ofthe charging device. In one example the charging cells are deployed inaccordance with a honeycomb packaging configuration. A charging cell maybe implemented using one or more coils that can each induce a magneticfield along an axis that is substantially orthogonal to the surface ofthe charging device and adjacent to the coil. In this description, acharging cell may refer to an element having one or more coils whereeach coil is configured to produce an electromagnetic field that isadditive with respect to the fields produced by other coils in thecharging cell, and directed along or proximate to a common axis.

In some implementations, a charging cell includes coils that are stackedalong a common axis and/or that overlap such that they contribute to aninduced magnetic field substantially orthogonal to the surface of thecharging device. In some implementations, a charging cell includes coilsthat are arranged within a defined portion of the surface of thecharging device and that contribute to an induced magnetic field withinthe substantially orthogonal portion of the surface of the chargingdevice associated with the charging cell. In some implementations,charging cells may be configurable by providing an activating current tocoils that are included in a dynamically-defined charging cell. Forexample, a charging device may include multiple stacks of coils deployedacross a surface of the charging device, and the charging device maydetect the location of a device to be charged and may select somecombination of stacks of coils to provide a charging cell adjacent tothe device to be charged. In some instances, a charging cell mayinclude, or be characterized as a single coil. However, it should beappreciated that a charging cell may include multiple stacked coilsand/or multiple adjacent coils or stacks of coils.

FIG. 1 illustrates an example of a charging cell 100 that may bedeployed and/or configured to provide a charging device. In thisexample, the charging cell 100 has a substantially hexagonal shape thatencloses one or more coils 102 constructed using conductors, wires orcircuit board traces that can receive a current sufficient to produce anelectromagnetic field in a power transfer area 104. In variousimplementations, some coils 102 may have a shape that is substantiallypolygonal, including the hexagonal charging cell 100 illustrated inFIG. 1. Other implementations may provide coils 102 that have othershapes. The shape of the coils 102 may be determined at least in part bythe capabilities or limitations of fabrication technology, and/or tooptimize layout of the charging cells on a substrate 106 such as aprinted circuit board substrate. Each coil 102 may be implemented usingwires, printed circuit board traces and/or other connectors in a spiralconfiguration. Each charging cell 100 may span two or more layersseparated by an insulator or substrate 106 such that coils 102 indifferent layers are centered around a common axis 108.

FIG. 2 illustrates the arrangement of power transfer areas providedacross a surface 200 of the charging device that employs multiple layersof charging cells configured in accordance with certain aspectsdisclosed herein. The charging device may be constructed from fourlayers of charging cells 202, 204, 206, 208. In FIG. 2, each powertransfer area provided by a charging cell in the first layer of chargingcells 202 is marked “L1”, each power transfer area provided by acharging cell in the second layer of charging cells 204 is marked “L2”,each power transfer area provided by a charging cell in the third layerof charging cells 206, 208 is marked “L3”, and each power transfer areaprovided by a charging cell in the first layer of charging cells 208 ismarked “L4”.

FIG. 3 illustrates a wireless transmitter 300 that may be provided in acharger base station. A controller 302 may receive a feedback signalfiltered or otherwise processed by a filter circuit 308. The controllermay control the operation of a driver circuit 304 that provides analternating current (AC) signal to a resonant circuit 306 that includesa capacitor 312 and inductor 314. The resonant circuit 306 may also bereferred to herein as a tank circuit, an LC tank circuit and/or as an LCtank, and the voltage 316 measured at an LC node 310 of the resonantcircuit 306 may be referred to as the tank voltage.

The wireless transmitter 300 may be used by a charging device todetermine if a compatible device has been placed on a surface of thecharging device. For example, the charging device may determine that acompatible device has been placed on the surface of the charging deviceby sending an intermittent test signal (active ping) through thewireless transmitter 300, where the resonant circuit 306 may receiveencoded signals when a compatible device responds to the test signal.The charging device may be configured to activate one or more coils inat least one charging cell after receiving a response signal defined bystandard, convention, manufacturer or application. In some examples, thecompatible device can respond to a ping by communicating received signalstrength such that the charging device can find an optimal charging cellto be used for charging the compatible device.

Passive ping techniques may use the voltage and/or current measured orobserved at the LC node 310 to identify the presence of a receiving coilin proximity to the charging pad of a device adapted in accordance withcertain aspects disclosed herein. In many conventional wireless chargertransmitters, circuits are provided to measure voltage at the LC node310 or the current in the network. These voltages and currents may bemonitored for power regulation purposes and/or to support communicationbetween devices. In the example illustrated in FIG. 3, voltage at the LCnode 310 is monitored, although it is contemplated that current mayadditionally or alternatively be monitored to support passive ping. Aresponse of the resonant circuit 306 to a passive ping (initial voltageV₀) may be represented by the voltage (V_(LC)) at the LC node 310, suchthat:

$\begin{matrix}{V_{LC} = {V_{0}e^{{- {(\frac{\omega}{2Q})}}t}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

According to certain aspects disclosed herein, coils in one or morecharging cells may be selectively activated to provide an optimalelectromagnetic field for charging a compatible device. In someinstances, coils may be assigned to charging cells, and some chargingcells may overlap other charging cells. In the latter instances, theoptimal charging configuration may be selected at the charging celllevel. In other instances, charging cells may be defined based onplacement of a device to be charged on a surface of the charging device.In these other instances, the combination of coils activated for eachcharging event can vary. In some implementations, a charging device mayinclude a driver circuit that can select one or more cells and/or one ormore predefined charging cells for activation during a charging event.

Phase Modulated Charging

One aspect of this disclosure relates to the use of a phase-modulatedwireless charger 400, an example of which is illustrated in FIG. 4. Adriver circuit 402 provides a charging current 410 to a resonant circuit404 that includes a capacitor (C_(p)) and an inductor (L_(p)). Thecharging current 410 may be substantially the same as the current in theinductor (i.e., the L_(p) current), although some portion of thecharging current 410 may be lost due to parasitic capacitance, or thelike. The charging current 410 alternates at a frequency that may beclosely matched to the resonant frequency of the resonant circuit 404 toimprove efficiency of power transfer. In accordance with certain aspectsof this disclosure, the level of power transferred through the resonantcircuit 404 to a receiving device may be controlled through phasemodulation of the charging current 410.

The timing diagram 420 illustrates certain aspects of phase modulationas applied to the charging current 410 in certain implementations. Phasemodulation enables fine control over the level of power delivery by thedriver circuit 402. The timing diagram 420 depicts three chargingperiods 422, 424 and 426 in which power is delivered at differentlevels, as indicated by the varying amplitude of the charging current410.

Phase control is obtained using a zero-crossing detector 406 and a phasemodulator 408 that responds to a phase control signal 418 provided by acontroller or other processor. The zero-crossing detector 406 is used toprovide timing information used by the phase modulator 408. In oneexample, the zero-crossing detector 406 may compare polarity of ameasurement signal 412 representing the current flowing to the resonantcircuit 404 with polarity of a delayed version of the measurement signal412, whereby a difference in polarity is detected when a zero-crossingoccurs in the measurement signal 412. The zero-crossing detector 406provides a zero-crossing signal 414 (ZC) that includes timinginformation identifying zero-crossings of the measurement signal 412. Inone example, the zero-crossing signal 414 includes an edge for eachzero-crossing of the measurement signal 412. Direction of transition ofthe edge may indicate positive-going or negative-going zero-crossings.In another example, the zero-crossing signal 414 includes a pulse foreach zero-crossing of the measurement signal 412.

The phase modulator 408 uses the zero-crossing signal 414 to generate aphase modulation signal 416. The phase modulation signal 416 may changethe phase of a modulated current that contributes to the chargingcurrent 410. The phase of the modulated current with respect to thephase of the current in the resonant circuit can cause an increase ordecrease in the charging current 410. In the first charging period 422,the phase modulation signal 416 is closely synchronized to thezero-crossing signal 414, and the effect of the modulated current isadditive over each cycle of the charging current 410. In this example,the driver circuit 402 provides maximum power transfer through theresonant circuit 404. In the second charging period 424, the phasemodulation signal 416 has a phase shift of 90° with respect to thezero-crossing signal 414, and the effect of the modulated current isadditive and subtractive on alternating quarter cycles. In this example,the driver circuit 402 provides 50% of the maximum available powerthrough the resonant circuit 404. In the third charging period 426, thephase modulation signal 416 has a phase shift with respect to thezero-crossing signal 414 that increases from 90° to 180° in thelast-depicted cycle 428. The effect of the modulated current is negativeover an increasing portion of each cycle of the charging current 410 anddriver circuit 402 provides power through the resonant circuit 404 thatdecreases from 50% of the maximum available power to no power transferor minimal power transfer.

In certain implementations, the zero-crossing signal 414 is provided asa digital signal that provides the timing needed by the phase modulator408 to add a phase-lead or phase-lag to the incoming zero-cross signalwhen indicated by the phase control signal 418. In one example, thedriver circuit 402 includes a half-bridge circuit. In one example, thephase control signal 418 is a multi-bit digital signal that indicatesthe amount of phase shift to be added to the zero-crossing signal 414 inorder to directly affect the amount of power that flows in the resonantcircuit 404 (i.e., Lp and Cp).

Resonant Pulse-Width Modulation

FIG. 5 illustrates an example of a PWM charger 500 and the timingdiagrams 600, 620 in FIG. 6 illustrate certain aspects of the operationof the PWM charger 500. One aspect of this disclosure relates to the useof a pulse-width modulation (PWM) charging system to modulate a chargingcurrent 510 provided to a resonant circuit 504. A driver circuit 502provides a charging current 510 to a resonant circuit 504 that includesa capacitor (C_(p)) and an inductor (L_(p)). The charging current 510may be substantially the same as the current in the inductor (i.e., theL_(p) current), although some portion of the charging current 510 may belost due to parasitic capacitance, or the like. The charging current 510alternates at a frequency that may be closely matched to the resonantfrequency of the resonant circuit 504 to improve efficiency of powertransfer. In accordance with certain aspects of this disclosure, thelevel of power transferred through the resonant circuit 504 to areceiving device may be controlled using PWM modulation to alter thecharging current 510.

The timing diagrams 600, 620 illustrate certain aspects of PWM asapplied to the charging current 510 in certain implementations. PWMenables fine control over the level of power delivery by the drivercircuit 502, although the timing diagrams 600, 620 depict a limitednumber of charging periods 602, 604, 606, 622, 624 and 626 in whichpower is delivered at different levels, as indicated by the varyingamplitude of the charging current 510.

The power provided in the charging current 510 may be controlled using azero-crossing detector 506 and a PWM circuit 508 that responds to acontrol signal 518 provided by a controller or other processor. Thezero-crossing detector 506 is used to provide timing information used bythe PWM circuit 508. In one example, the zero-crossing detector 506 maycompare the polarity of a measurement signal 512 representing thecurrent flowing to the resonant circuit 504 with the polarity of adelayed version of the measurement signal 512, whereby a difference inpolarity is detected when a zero-crossing occurs in the measurementsignal 512. The zero-crossing detector 506 provides a zero-crossingsignal 514 (ZC) that includes timing information identifyingzero-crossings of the measurement signal 512. In one example, thezero-crossing signal 514 includes an edge for each zero-crossing of themeasurement signal 512. Direction of transition of the edge may indicatepositive-going or negative-going zero-crossings. In another example, thezero-crossing signal 514 includes a pulse for each zero-crossing of themeasurement signal 512.

The PWM circuit 508 uses the zero-crossing signal 514 to generate a PWMsignal 516. The PWM signal 516 may control the contribution of energy tothe charging current 510. In one example, pulses in the PWM signal 516are used to gate a current that is provided to a power inverter circuitthat produces an alternating output used to provide the charging current510.

In the first charging period 602, 622, the PWM signal 516 includespulses that match the duration of a half-cycle of the charging current510, and provides a charging current 510 with maximum (100%) power. Inthis example, the driver circuit 502 provides maximum power transferthrough the resonant circuit 504. In the second charging period 604,624, the PWM signal 516 includes pulses that have a duration ofapproximately half the duration of a half-cycle of the charging current510, and the resultant charging current 510 with provides 50% of themaximum available power when provided to the resonant circuit 504. Inthe third charging period 606, 626, the PWM signal 516 includes pulsesthat decrease, initially having a duration of approximately half theduration of a half-cycle of the charging current 510, and decreasing toalmost an absence of a pulse. The driver circuit 502 provides powerthrough the resonant circuit 504 that decreases from 50% of the maximumavailable power to no or minimal power transfer.

The timing of the pulses in the PWM signal 516 may be selected based onthe method of generating the charging current 510 used in the drivercircuit 502. In the example illustrated by the first timing diagram 600of FIG. 6, each pulse is initiated at a zero crossing and has a durationthat may be determined by the width control signal 518. The widthcontrol signal 518 may be provided as a multi-bit digital signal thatconfigures a programmable delay circuit or selects an out of a delayline to provide a delay that determines the duration of a pulse in thewidth control signal 518.

In the example illustrated by the second timing diagram 620 of FIG. 6,each pulse in the PWM signal 516 is centered on the mid-point of acorresponding pulse in the zero-crossing signal 514. In other words, thecenter of each pulse is midway between zero crossings of the measurementsignal 512. The duration of these pulses may be determined by the widthcontrol signal 518. The width control signal 518 may be provided as amulti-bit digital signal that configures a programmable delay circuit orselects an out of a delay line to provide a delay that determines theduration of a pulse in the width control signal 518. The location of thepulses may be configured using counters, delay lines, lookup tablesand/or other circuits. Centering the pulses in the PWM signal 516between zero crossings of the measurement signal 512 can lowerdistortion of the AC signal in the charging current 510.

In some implementations, resonant pulse width modulation may use adetected zero-crossing as a temporal reference to initiate a PWM drivecycle. In one example, a timer may be started to control with width ofthe pulse. In another example, a delay circuit may be used to controlwith width of the pulse. The charging current 510 flowing in theresonant circuit 504 is controlled by the width of the pulse.

In some implementations, PWM may be used to control the charging current510 flowing in the resonant circuit 504 without zero-crossingsynchronization. Accordingly, a current measurement circuit and azero-crossing detector 506 may not be necessary, provided otherinformation is known, including the values of L_(p) and C_(p), forexample.

Resonant Class-D Wireless Transmitter

FIG. 7 illustrates an example of a wireless charging system 700 thatemploys a class-D wireless transmitter 702 provided in accordance withcertain aspects disclosed herein. The timing diagram 800 in FIG. 8illustrate certain aspects of the operation of the class-D wirelesstransmitter 702. The class-D wireless transmitter 702 includes a class-Damplifier that operates as a switching amplifier. The class-D wirelesstransmitter 702 generates a first signal that switches between voltagerails at a first frequency. The first signal is modulated by a secondlower-frequency signal. In the illustrated example, the first signal ispulse-width modulated to obtain a PWM signal 718.

The PWM signal 718 is provided to a driver circuit 704 that generates acharging current to drive a resonant circuit 706 that includes an LCtank circuit including a capacitor (C_(p)) and an inductor (L_(p)). Thecharging current may be substantially the same as the current in theinductor (i.e., the L_(p) current 802). The resonant circuit 706operates as a low-pass filter that converts the high frequency PWMsignal 718 to obtain an amplified version of the modulating signal,which may be a sine wave. The PWM controller 710 may be operated tocontrol the peak amplitude of the L_(p) current 802 using cumulativescaling in order to control the power transmitted to a wireless receiver730. For example, wider pulses in the PWM signal 718 may correspond topeaks in the L_(p) current 802 amplitude.

The power provided by the driver circuit 704 may be controlled using azero-crossing detector 708 and the PWM controller 710, which may respondto a control signal 720 provided by a controller or other processor. ThePWM controller 710 receives a sinusoidal signal from a reference source712 that provides a carrier signal that can be PWM modulated. Thezero-crossing detector 708 is used to provide timing information used bythe PWM controller 710. In one example, the zero-crossing detector 708may compare the polarity of a measurement signal 714 representing thecurrent flowing to the resonant circuit 706 with the polarity of adelayed version of the measurement signal 714, whereby a difference inpolarity is detected when a zero-crossing occurs in the measurementsignal 714. The zero-crossing detector 708 provides a zero-crossingsignal 716 (ZCS) that includes timing information identifyingzero-crossings of the measurement signal 714. In one example, thezero-crossing signal 716 includes an edge for each zero-crossing of themeasurement signal 714. Direction of transition of the edge may indicatepositive-going or negative-going zero-crossings. In another example, thezero-crossing signal 716 includes a pulse for each zero-crossing of themeasurement signal 714. The PWM controller 710 may use the zero-crossingsignal 716 to generate a PWM signal 718, in which the PWM signal 718 isin phase alignment with the L_(p) current 802.

Zero-Crossing Slotted Foreign Object Detection

Slotted foreign object detection may be used to detect a foreign object(FO) on the surface of a wireless charging device. A driver circuit inthe wireless charging device is periodically turned off for a shortperiod of time, which may be referred to as a slot, during which theenergy in a resonant circuit driven by the driver circuit is allowed todecay. The Q factor of the resonant circuit can be determined bymeasuring the rate of decay. A high sample rate is typically required toaccurately measure the AC waveform in the tank circuit without aliasingor artifacts that may spoil the measurement accuracy of the Q factor.The sample rate can be a factor of ten to twenty times the frequency ofthe current in the resonant circuit, and generally requires the use of afast and expensive analog-to-digital converter (ADC).

In certain aspects of the disclosure, a zero-crossing detector is usedto provide timing information that permits a low-cost ADC to reliablyobtain an accurate measurement of the voltage at the same point in eachcycle of the AC waveform in the resonant circuit, during a slot providedfor foreign object detection. Zero crossing slotted foreign objectdetection can be used to detect the zero crossing of either the voltageand/or the current in the resonant circuit. The detection of the zerocrossing starts a hold-off timer that triggers a sample and hold circuitin the ADC. In one example, the hold-off timer triggers the sample andhold circuit after a quarter cycle of the AC waveform in the resonantcircuit. In this example, the ADC reads a sample taken at the peak ofthe AC wave. A sample frequency that is less than the fundamentalfrequency of the AC waveform can be used.

FIG. 9 includes timing diagrams 900, 920 that illustrate certain aspectsof a zero-crossing, slotted foreign object detection. A measurement slot906, 926 is provided between periods 904, 908 or 924, 928 of normalcharging operation. The first timing diagram 900 relates to an exampleof a signal 902 representing energy, voltage or current in the resonantcircuit when no foreign object is present, and the slow decay 912 in thesignal 902 corresponds to a resonant circuit with a high Q factor. Thesecond timing diagram 920 relates to an example of a signal 922representing energy, voltage or current in the resonant circuit when aforeign object 1030 (see FIG. 10) is present, and the decay 932corresponds to a resonant circuit with a low Q factor. A zero-crossing,slotted foreign object detection technique according to certain aspectsof the disclosure uses sample points 914, 934 identified based ondetected zero crossings identified by a zero-crossing signal 910, 930.

FIG. 10 illustrates an example of a wireless charging system 1000 thatemploys zero-crossing detection to obtain measurements 1028 at one ormore points in each cycle of current or voltage in a resonant circuit1004. In one example, the measurements may be used for slotted foreignobject detection in accordance with certain aspects disclosed herein.The wireless charging system 1000 includes a driver circuit 1002 thatgenerates a charging current to drive a resonant circuit 1004 thatincludes an LC tank circuit including a capacitor (C_(p)) and aninductor (L_(p)). The charging current may be substantially the same asthe current in the inductor. In some implementations, a voltagemeasurement signal 1006 representative of the voltage across theresonant circuit 1004 is provided to a first zero-crossing detector1012. The first zero-crossing detector 1012 produces an output 1016(ZVS) indicating the timing of zero-crossings of the voltage across theresonant circuit 1004. In some implementations, a current measurementsignal 1008 representative of the current in the resonant circuit 1004is provided to a second zero-crossing detector 1014. The secondzero-crossing detector 1014 produces an output 1018 (ZCS) indicating thetiming of zero-crossings of the current in the resonant circuit 1004.

A capture timing circuit 1020 may be used to track zero crossings anddetermine or manage the sample and hold circuit 1024. In one example,the capture timing circuit 1020 may include or use a hold-off timer 1022that can locate the peak amplitude of the voltage or current across theresonant circuit 1004 that occurs after period of time corresponding toa half cycle of the resonant circuit 1004. In other examples, thecapture timing circuit 1020 may include or use a hold-off timer 1022that can locate one or more points of the voltage or current across theresonant circuit 1004. The sample and hold circuit 1024 provides anoutput digitized by the ADC 1026 to obtain a measurement 1028. Themeasurement 1028 may be used to track the rate of decay of the energy inthe resonant circuit 1004.

Zero-Crossing Amplitude Shift Key Demodulation

The measurements obtained using the zero-crossing detection techniquesillustrated in FIG. 10 may be used for Amplitude Shift Keying (ASK)demodulation. ASK modulation is commonly used to carry messages definedby the Qi protocol, which is used for wirelessly interconnecting a powertransmitter to a power receiver. The Qi protocol permits the powerreceiver to control the power transmitter wirelessly. The measurements1028 obtained at one or more points in each cycle of current or voltagein a resonant circuit 1004 may be used for ASK demodulation. One or morezero-crossing detectors 1012, 1014 provide reference timing for samplingvoltage or current associated with the resonant circuit 1004. Sampleddata can be used to extract the ASK data that is modulated on thecarrier power signal by the receiving device.

Data can be extracted from signals that have much higher frequenciesthan the sampling frequency when zero cross detection is used to providetiming for sampling. In some instances, sampling can be performed at thefundamental frequency of the current or voltage associated with theresonant circuit 1004, or at double the frequency of the current orvoltage associated with the resonant circuit 1004. Conventional samplingcircuits operate at ten times the fundamental frequency of the currentor voltage associated with the resonant circuit 1004 or more to avoidaliasing and other distortion artifacts.

In one example, ASK demodulation is performed using measurements ofvoltage captured using timing provided by the output 1016 (ZVS) of thefirst zero-crossing detector 1012 to time the trigger of a sample andhold circuit 1024. In another example, ASK demodulation is performedusing measurements of current captured using timing provided by theoutput 1018 (ZCS) of the second zero-crossing detector 1014 to time thetrigger of a sample and hold circuit 1024. ASK demodulation can beperformed using a single sample taken at the peak of a cycle of voltageor current. Zero-crossing ASK demodulation can reject any communicationschannels that may be in the same domain, provided the phase and/orfrequency of the interfering carrier is different from the targetcarrier.

FIGS. 11 and 12 illustrate an example of a wireless charging system 1200that employs zero-crossing detection to support phase-based ASKdemodulation. Referring to the timing diagram 1100 of FIG. 11,zero-crossing phase demodulation includes detecting the phase differencebetween zero-volt crossings of the voltage 1108 and the current 1106 inthe resonant circuit 1204. Phase shifts between the voltage 1108 and thecurrent 1106 may correspond to different modulation levels 1102 when thepower receiving device 1206 uses ASK modulation to encode data throughload or resonance shift. A digital phase detector 1212 can determine thephase difference between a current zero-crossing signal 1220 (ZCS) and avoltage zero-crossing signal 1222 (ZVS) provided by correspondingzero-crossing detector circuits 1208, 1210 respectively. Phasedifferences can be measured at one or more points in each cycle ofcurrent or voltage in a resonant circuit 1204. The wireless chargingsystem 1200 includes a driver circuit 1202 that generates a chargingcurrent 1104 to drive the resonant circuit 1204, which includes acapacitor (C_(p)) and an inductor (L_(p)). The charging current 1104 maybe substantially the same as the current in the inductor. In someimplementations, a voltage measurement signal 1218 representative of thevoltage across the resonant circuit 1204 is provided to a firstzero-crossing detector 1210. The first zero-crossing detector 1210produces an output (ZVS) indicating the timing of zero-crossings of thevoltage across the resonant circuit 1204. A current measurement signal1216 representative of the current in the resonant circuit 1204 isprovided to a second zero-crossing detector 1208. The secondzero-crossing detector 1208 produces an output (ZCS) indicating thetiming of zero-crossings of the current in the resonant circuit 1204.

The phase detector circuit 1212 provides a signal representative of thephase difference between the current zero-crossing signal 1220 (ZCS) andthe voltage zero-crossing signal 1222 (ZVS) to an ASK demodulator 1214.

Example of a Processing Circuit

FIG. 13 illustrates an example of a hardware implementation for anapparatus 1300 that may be incorporated in a charging device or in areceiving device that enables a battery to be wirelessly charged. Insome examples, the apparatus 1300 may perform one or more functionsdisclosed herein. In accordance with various aspects of the disclosure,an element, or any portion of an element, or any combination of elementsas disclosed herein may be implemented using a processing circuit 1302.The processing circuit 1302 may include one or more processors 1304 thatare controlled by some combination of hardware and software modules.Examples of processors 1304 include microprocessors, microcontrollers,digital signal processors (DSPs), SoCs, ASICs, field programmable gatearrays (FPGAs), programmable logic devices (PLDs), state machines,sequencers, gated logic, discrete hardware circuits, and other suitablehardware configured to perform the various functionality describedthroughout this disclosure. The one or more processors 1304 may includespecialized processors that perform specific functions, and that may beconfigured, augmented or controlled by one of the software modules 1316.The one or more processors 1304 may be configured through a combinationof software modules 1316 loaded during initialization, and furtherconfigured by loading or unloading one or more software modules 1316during operation.

In the illustrated example, the processing circuit 1302 may beimplemented with a bus architecture, represented generally by the bus1310. The bus 1310 may include any number of interconnecting buses andbridges depending on the specific application of the processing circuit1302 and the overall design constraints. The bus 1310 links togethervarious circuits including the one or more processors 1304, and storage1306. Storage 1306 may include memory devices and mass storage devices,and may be referred to herein as computer-readable media and/orprocessor-readable media. The storage 1306 may include transitorystorage media and/or non-transitory storage media.

The bus 1310 may also link various other circuits such as timingsources, timers, peripherals, voltage regulators, and power managementcircuits. A bus interface 1308 may provide an interface between the bus1310 and one or more transceivers 1312. In one example, a transceiver1312 may be provided to enable the apparatus 1300 to communicate with acharging or receiving device in accordance with a standards-definedprotocol. Depending upon the nature of the apparatus 1300, a userinterface 1318 (e.g., keypad, display, speaker, microphone, joystick)may also be provided, and may be communicatively coupled to the bus 1310directly or through the bus interface 1308.

A processor 1304 may be responsible for managing the bus 1310 and forgeneral processing that may include the execution of software stored ina computer-readable medium that may include the storage 1306. In thisrespect, the processing circuit 1302, including the processor 1304, maybe used to implement any of the methods, functions and techniquesdisclosed herein. The storage 1306 may be used for storing data that ismanipulated by the processor 1304 when executing software, and thesoftware may be configured to implement any one of the methods disclosedherein.

One or more processors 1304 in the processing circuit 1302 may executesoftware. Software shall be construed broadly to mean instructions,instruction sets, code, code segments, program code, programs,subprograms, software modules, applications, software applications,software packages, routines, subroutines, objects, executables, threadsof execution, procedures, functions, algorithms, etc., whether referredto as software, firmware, middleware, microcode, hardware descriptionlanguage, or otherwise. The software may reside in computer-readableform in the storage 1306 or in an external computer-readable medium. Theexternal computer-readable medium and/or storage 1306 may include anon-transitory computer-readable medium. A non-transitorycomputer-readable medium includes, by way of example, a magnetic storagedevice (e.g., hard disk, floppy disk, magnetic strip), an optical disk(e.g., a compact disc (CD) or a digital versatile disc (DVD)), a smartcard, a flash memory device (e.g., a “flash drive,” a card, a stick, ora key drive), RAM, ROM, a programmable read-only memory (PROM), anerasable PROM (EPROM) including EEPROM, a register, a removable disk,and any other suitable medium for storing software and/or instructionsthat may be accessed and read by a computer. The computer-readablemedium and/or storage 1306 may also include, by way of example, acarrier wave, a transmission line, and any other suitable medium fortransmitting software and/or instructions that may be accessed and readby a computer. Computer-readable medium and/or the storage 1306 mayreside in the processing circuit 1302, in the processor 1304, externalto the processing circuit 1302, or be distributed across multipleentities including the processing circuit 1302. The computer-readablemedium and/or storage 1306 may be embodied in a computer programproduct. By way of example, a computer program product may include acomputer-readable medium in packaging materials. Those skilled in theart will recognize how best to implement the described functionalitypresented throughout this disclosure depending on the particularapplication and the overall design constraints imposed on the overallsystem.

The storage 1306 may maintain and/or organize software in loadable codesegments, modules, applications, programs, etc., which may be referredto herein as software modules 1316. Each of the software modules 1316may include instructions and data that, when installed or loaded on theprocessing circuit 1302 and executed by the one or more processors 1304,contribute to a run-time image 1314 that controls the operation of theone or more processors 1304. When executed, certain instructions maycause the processing circuit 1302 to perform functions in accordancewith certain methods, algorithms and processes described herein.

Some of the software modules 1316 may be loaded during initialization ofthe processing circuit 1302, and these software modules 1316 mayconfigure the processing circuit 1302 to enable performance of thevarious functions disclosed herein. For example, some software modules1316 may configure internal devices and/or logic circuits 1322 of theprocessor 1304, and may manage access to external devices such as atransceiver 1312, the bus interface 1308, the user interface 1318,timers, mathematical coprocessors, and so on. The software modules 1316may include a control program and/or an operating system that interactswith interrupt handlers and device drivers, and that controls access tovarious resources provided by the processing circuit 1302. The resourcesmay include memory, processing time, access to a transceiver 1312, theuser interface 1318, and so on.

One or more processors 1304 of the processing circuit 1302 may bemultifunctional, whereby some of the software modules 1316 are loadedand configured to perform different functions or different instances ofthe same function. The one or more processors 1304 may additionally beadapted to manage background tasks initiated in response to inputs fromthe user interface 1318, the transceiver 1312, and device drivers, forexample. To support the performance of multiple functions, the one ormore processors 1304 may be configured to provide a multitaskingenvironment, whereby each of a plurality of functions is implemented asa set of tasks serviced by the one or more processors 1304 as needed ordesired. In one example, the multitasking environment may be implementedusing a timesharing program 1320 that passes control of a processor 1304between different tasks, whereby each task returns control of the one ormore processors 1304 to the timesharing program 1320 upon completion ofany outstanding operations and/or in response to an input such as aninterrupt. When a task has control of the one or more processors 1304,the processing circuit is effectively specialized for the purposesaddressed by the function associated with the controlling task. Thetimesharing program 1320 may include an operating system, a main loopthat transfers control on a round-robin basis, a function that allocatescontrol of the one or more processors 1304 in accordance with aprioritization of the functions, and/or an interrupt driven main loopthat responds to external events by providing control of the one or moreprocessors 1304 to a handling function.

In one implementation, the apparatus 1300 includes or operates as awireless charging apparatus that has a battery charging power sourcecoupled to a charging circuit, a plurality of charging cells and acontroller, which may be included in one or more processors 1304. Theplurality of charging cells may be configured to provide a chargingsurface. At least one transmitting coil may be configured to direct anelectromagnetic field through a charge transfer area of each chargingcell. The apparatus 1300 may include a resonant circuit comprising atransmitting coil, a driver circuit configured to provide a chargingcurrent to the resonant circuit, a zero-crossing detector configured toprovide a zero-crossing signal that includes edges corresponding totransitions of a voltage measured across the resonant circuit through azero volt level or corresponding to transitions of a current in theresonant circuit through a zero ampere level, and an ASK demodulator.The ASK demodulator may be configured to receive a plurality of samplesof voltage or current in the resonant circuit captured at timesdetermined by the edges included in the zero-crossing signal, anddemodulate a modulated signal obtained from the charging current usingthe plurality of samples of voltage or current in the resonant circuit.

In one example, the zero volt level corresponds to a current amplitudemidway between maximum and minimum amplitudes of an AC measured in theresonant circuit. The zero volt level may correspond to a voltage levelmidway between maximum and minimum amplitudes of an AC voltage measuredacross the resonant circuit.

In certain examples, each of the plurality of samples of voltage orcurrent in the resonant circuit is captured following after a delay thatfollows an edge in the zero-crossing signal. The delay may be calculatedto cause sampling of the voltage or current when the voltage or currenthas a maximum amplitude.

In one example, the ASK demodulator is further configured to use aseries of samples of voltage in the resonant circuit to demodulate themodulated signal. The ASK demodulator may be further configured to use aseries of samples of current in the resonant circuit to demodulate themodulated signal. The ASK demodulator may be further configured todetermine phase differences between a series samples of voltage in theresonant circuit and a corresponding series of samples of current in theresonant circuit, and demodulate the modulated signal based on the phasedifferences.

In some implementations, the storage 1306 maintains instructions andinformation where the instructions are configured to provide a chargingcurrent to a resonant circuit when a receiving device is present on asurface of the wireless charging device, provide a zero-crossing signalthat includes edges corresponding to transitions of a voltage measuredacross the resonant circuit through a zero volt level or to transitionsof a current in the resonant circuit through a zero ampere level,capture a plurality of samples of voltage or current in the resonantcircuit at times determined by the edges included in the zero-crossingsignal, and demodulate a modulated signal obtained from the chargingcurrent using the plurality of samples of voltage or current in theresonant circuit, wherein the modulated signal is modulated using ASK.

In one example, the zero volt level corresponds to a current amplitudemidway between maximum and minimum amplitudes of an AC measured in theresonant circuit. The zero volt level may correspond to a voltage levelmidway between maximum and minimum amplitudes of an AC voltage measuredacross the resonant circuit.

In certain examples, each of the plurality of samples of voltage orcurrent in the resonant circuit is captured following after a delay thatfollows an edge in the zero-crossing signal. The delay may be calculatedto cause sampling of the voltage or current when the voltage or currenthas a maximum amplitude.

In one example, a series of samples of voltage in the resonant circuitmay be used to demodulate the modulated signal. In another example, aseries of samples of current in the resonant circuit may be used todemodulate the modulated signal. In another example, the charging devicemay determine phase differences between a series samples of voltage inthe resonant circuit and a corresponding series of samples of current inthe resonant circuit, and demodulate the modulated signal based on thephase differences.

FIG. 14 is a flowchart 1400 illustrating a method for operating acharging device in accordance with certain aspects of this disclosure.The method may be performed at the charging device. At block 1402, thecharging device may provide a charging current to a resonant circuitwhen a receiving device is present on a surface of the wireless chargingdevice. At block 1404, the charging device may provide a zero-crossingsignal that includes edges corresponding to transitions of a voltagemeasured across the resonant circuit through a zero volt level or totransitions of a current in the resonant circuit through a zero amperelevel. At block 1406, the charging device may capture a plurality ofsamples of voltage or current in the resonant circuit at timesdetermined by the edges included in the zero-crossing signal. At block1408, the charging device may demodulate a modulated signal obtainedfrom the charging current using the plurality of samples of voltage orcurrent in the resonant circuit, wherein the modulated signal ismodulated using ASK.

In one example, the zero volt level corresponds to a current amplitudemidway between maximum and minimum amplitudes of an AC measured in theresonant circuit. The zero volt level may correspond to a voltage levelmidway between maximum and minimum amplitudes of an AC voltage measuredacross the resonant circuit.

In certain examples, each of the plurality of samples of voltage orcurrent in the resonant circuit is captured following after a delay thatfollows an edge in the zero-crossing signal. The delay may be calculatedto cause sampling of the voltage or current when the voltage or currenthas a maximum amplitude.

In one example, a series of samples of voltage in the resonant circuitmay be used to demodulate the modulated signal. In another example, aseries of samples of current in the resonant circuit may be used todemodulate the modulated signal. In another example, the charging devicemay determine phase differences between a series samples of voltage inthe resonant circuit and a corresponding series of samples of current inthe resonant circuit, and demodulate the modulated signal based on thephase differences.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but is to be accorded the full scope consistentwith the language claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.” Unless specifically statedotherwise, the term “some” refers to one or more. All structural andfunctional equivalents to the elements of the various aspects describedthroughout this disclosure that are known or later come to be known tothose of ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed by the claims. Moreover,nothing disclosed herein is intended to be dedicated to the publicregardless of whether such disclosure is explicitly recited in theclaims. No claim element is to be construed under the provisions of 35U.S.C. § 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for” or, in the case of a method claim, theelement is recited using the phrase “step for.”

What is claimed is:
 1. A method for operating a wireless chargingdevice, comprising: providing a charging current to a resonant circuitwhen a receiving device is present on a surface of the wireless chargingdevice; providing a zero-crossing signal that includes edgescorresponding to transitions of a voltage measured across the resonantcircuit through a zero volt level or to transitions of a current in theresonant circuit through a zero ampere level; capturing a plurality ofsamples of voltage or current in the resonant circuit at timesdetermined by the edges included in the zero-crossing signal, whereinthe plurality of samples of voltage or current in the resonant circuitis captured by sampling the voltage or current at a fundamentalfrequency of the current or voltage in the resonant circuit; anddemodulating a modulated signal obtained from the charging current usingthe plurality of samples of voltage or current in the resonant circuit,wherein the modulated signal is modulated using Amplitude Shift Keying(ASK).
 2. The method of claim 1, wherein the zero volt level correspondsto a current amplitude midway between maximum and minimum amplitudes ofan alternating current (AC) measured in the resonant circuit.
 3. Themethod of claim 1, wherein the zero volt level corresponds to a voltagelevel midway between maximum and minimum amplitudes of an AC voltagemeasured across the resonant circuit.
 4. The method of claim 1, furthercomprising: capturing each of the plurality of samples of voltage orcurrent in the resonant circuit following after a delay that follows anedge in the zero-crossing signal.
 5. The method of claim 4, wherein thedelay is calculated to cause sampling of the voltage or current when thevoltage or current has a maximum amplitude.
 6. The method of claim 1,further comprising: using a series of samples of voltage in the resonantcircuit to demodulate the modulated signal.
 7. The method of claim 1,further comprising: using a series of samples of current in the resonantcircuit to demodulate the modulated signal.
 8. The method of claim 1,further comprising: determining phase differences between a series ofsamples of voltage in the resonant circuit and a corresponding series ofsamples of current in the resonant circuit; and demodulating themodulated signal based on the phase differences.
 9. A charging device,comprising: a resonant circuit comprising a transmitting coil; a drivercircuit configured to provide a charging current to the resonantcircuit; a zero-crossing detector configured to provide a zero-crossingsignal that includes edges corresponding to transitions of a voltagemeasured across the resonant circuit through a zero volt level orcorresponding to transitions of a current in the resonant circuitthrough a zero ampere level; and an Amplitude Shift Keying demodulatorconfigured to: receive a plurality of samples of voltage or current inthe resonant circuit captured at times determined by the edges includedin the zero-crossing signal, wherein the plurality of samples of voltageor current in the resonant circuit is captured by sampling the voltageor current at a fundamental frequency of the current or voltage in theresonant circuit; and demodulate a modulated signal obtained from thecharging current using the plurality of samples of voltage or current inthe resonant circuit.
 10. The charging device of claim 9, wherein thezero volt level corresponds to a current amplitude midway betweenmaximum and minimum amplitudes of an alternating current (AC) measuredin the resonant circuit.
 11. The charging device of claim 9, wherein thezero volt level corresponds to a voltage level midway between maximumand minimum amplitudes of an AC voltage measured across the resonantcircuit.
 12. The charging device of claim 9, wherein each of theplurality of samples of voltage or current in the resonant circuit iscaptured following after a delay that follows an edge in thezero-crossing signal.
 13. The charging device of claim 12, wherein thedelay is calculated to cause sampling of the voltage or current when thevoltage or current has a maximum amplitude.
 14. The charging device ofclaim 9, wherein the Amplitude Shift Keying demodulator is furtherconfigured to: use a series of samples of voltage in the resonantcircuit to demodulate the modulated signal.
 15. The charging device ofclaim 9, wherein the Amplitude Shift Keying demodulator is furtherconfigured to: use a series of samples of current in the resonantcircuit to demodulate the modulated signal.
 16. The charging device ofclaim 9, wherein the Amplitude Shift Keying demodulator is furtherconfigured to: determine phase differences between a series of samplesof voltage in the resonant circuit and a corresponding series of samplesof current in the resonant circuit; and demodulate the modulated signalbased on the phase differences.
 17. A non-transitory processor-readablestorage medium having instructions stored thereon which, when executedby at least one processor of a processing circuit, cause the processingcircuit to: provide a charging current to a resonant circuit when areceiving device is present on a surface of a wireless charging device;provide a zero-crossing signal that includes edges corresponding totransitions of a voltage measured across the resonant circuit through azero volt level or to transitions of a current in the resonant circuitthrough a zero ampere level; capture a plurality of samples of voltageor current in the resonant circuit at times determined by the edgesincluded in the zero-crossing signal, wherein the plurality of samplesof voltage or current in the resonant circuit is captured by samplingthe voltage or current at a fundamental frequency of the current orvoltage in the resonant circuit; and demodulate a modulated signalobtained from the charging current using the plurality of samples ofvoltage or current in the resonant circuit, wherein the modulated signalis modulated using Amplitude Shift Keying (ASK).
 18. The non-transitoryprocessor-readable storage medium of claim 17, wherein the instructionsfurther cause the processing circuit to: capture each of the pluralityof samples of voltage or current in the resonant circuit following aftera delay that follows an edge in the zero-crossing signal, wherein thedelay is calculated to cause sampling of the voltage or current when thevoltage or current has a maximum amplitude.
 19. The non-transitoryprocessor-readable storage medium of claim 17, wherein the instructionsfurther cause the processing circuit to: determine phase differencesbetween a series of samples of voltage in the resonant circuit and acorresponding series of samples of current in the resonant circuit; anddemodulate the modulated signal based on the phase differences.